Hull for low drag boats

ABSTRACT

A hull for low drag boats has, from bow to stern, a deep V-shaped keel (10) whose dead-rise angle decreases up to a flattened bottom (30) in the stern, and sides (20) comprising side chines (200) protruding downward from the flattened bottom that is not inclined upwards.

TECHNICAL FIELD

The present invention relates to a hull for low drag boats.

The main goal in ship design is to reduce the surface friction and theextension of the wetted surface that cause the drag. The main aims areto reduce the fuel cost affecting maritime transportation and hinderingmarine business development, as well as to increase ship speed ingeneral.

BACKGROUND ART

In the past considerable efforts to increase the ship speed have beenmade. For example, drag is reduced in planing hull boats by raisingtheir bow, decreasing thereby the surface of the hull which is incontact with water. Other boats with planing hulls, includes a steppedconfiguration which is provided for a ventilation of the hull, in orderto reduce the actual surface area which is in contact with water.Another way is to inject air under the hull or to form an air cushionbeneath it, or to direct bubbles in some areas of the hull itself.

FR 2,604,412 describes a hull for fast boats where there is a front hullpart having a deep V-shaped profile with a gradually increasing anglefrom bow to stern, and a rear hull part having sides provided withlongitudinal floats. In the bottom of the hull a volume is delimited bythe bow, the longitudinal floats and a drain valve, volume in which asupporting air cushion is formed thanks to centrifugal fans that sendair under pressure to nozzles in the hull surface. The boat described inthe patent FR 2 604 412 is within the field of the so-called Air SupportVessels (ASV), which includes also the hovercrafts. However, the airunder pressure is a medium that supports the hull less than the waterdoes; anyway to blow air under the hull means using a lot of energy withconsequent drawbacks.

Many documents describe hulls in which air is blown in the surroundingliquid medium with a result of forming bubbles for drag reduction: seefor example inventions described by U.S. Pat. No. 6,186,085, U.S.2002/0014192 A1, WO 2011/161,187 A1, JP10175587A. The blowing of air inthe interface between the hull and the water causes the formation offoam. Even if the foam does not support the hull, the foam provides theadvantage of breaking the boundary layer formed by the surface frictionand generate a turbulent flow that replaces the laminar flow. Generallythe hulls described in the above mentioned patents require an increasein power consumption, i.e. the power which serves to blow air under thehull.

European patent EP 1 501 718 of the same applicant describes a mono-hullin which a midship cross section on the waterline divides a front hullpart, including a central keel, from a rear hull part having sidesprovided with chines that limit a progressively raising bottom in aftdirection, i.e. inclined upward, from the central keel aft toward thewaterline. The front hull part has a deep V-shaped profile, with thecentral keel extending in after direction with the angle formed by theV-shaped profile increasing up to create a flattened bottom.

The hull described by EP 1 501 718 should receive in its bottom the bowwave system and then recover some of the power consumed in forming thatbow wave system in order to increase its hydrodynamic support andmaintain a nearly horizontal trim.

On the boat described in the European patent EP 1 501 718, if the bow israised, the boat cannot generate the wave formation that is recovered inthe rear of the boat between the side chines, and which allows the hullto be hydrodynamically sustained.

Further, if the bow is raised, it does not produce the turbulent flowand the foam that is interposed between the hull surface and the water,which has been proved to be essential for greatly reducing dragfriction.

Again with reference to the bow raising, the feature of the bottom thatraises, as illustrated, described and claimed in mono-hull according tothe cited patent EP 1 501 718, is associated with the increased forwardspeed of the boat, which causes the hull to rotate around its center ofgravity and results in taking immediately a trim such as to eliminatethe angle that the bottom forms with the water surface when the boat isstationary. In other words, the raising bottom is positionedhorizontally with the consequence of raising the bow.

This trim is also accentuated by the fact that a thruster, usuallypositioned in the stern area, “empties” the rear hull part between theside chines during its normal operation. Therefore it should beunderstood that, because of this “emptying” of the rear hull part, thebow raises further and, then, as mentioned above, does not perform itsfunction, i.e. to generating the turbulent flow and the foam associatedherewith.

The behavior of the known hull 2 is represented in a drawing made from aphotograph (FIG. 1), in which a model is seen while being tested in towtowards the end of 2008 in the Froude's tank of the University ofTrieste. The hull 2 is of the mono-catamaran type. The known boat has akeel in the front hull part and sides with side chines in the rear hullpart. The deep V-shaped keel begins to flatten aft of midship. Thebottom aft of midship between the side chines raises upwards, that is tosay, it is inclined upwards from the horizon.

The model corresponds to an actual hull of 24 m and 70 t advancing at 24knots. It is noted that the keel is raised, but it was observed duringthe various series of tests that already at 10 knots the model assumedthat trim. In other words the profile of the rear hull part puts theboat even at low speeds in a stern-down trim. With this trim the keelthat is just little immersed, no longer performs its function ofgenerating the wave formation and the foam.

Briefly, it should be understood that the drawbacks caused by theraising bow are determined mainly by the fact that the bottom, thesurface of which is predominant in the boat raising, is on the rise. Theboat takes on a trim dependent on this rise angle of the bottom as theboat rotates around its center of gravity with a result of raising thebow.

To overcome these drawbacks caused by the raising bow, a first solutionwould be to make the bow very deep, so that the bow can generate thewave formation and the foam even when the bottom of the rear hull partis positioned horizontally. In this solution however the drag at lowspeed would be too high when the bow is deep in the water.

Another solution could be to provide the hull with large flapsprotruding from the stern. For this solution however, the flaps wouldsignificantly increase the drag especially at high speeds.

The solution to the above mentioned drawbacks is achieved according tothe present invention by a not raising bottom, i.e. a bottom that is notinclined upwards with respect to the horizon, aft of the midship crosssection.

SUMMARY OF THE INVENTION

Therefore, the present invention provides a hull for low drag boats,having from bow to stern a deep V-shaped keel whose dead-rise angledecreases up to a flattened bottom in the stern, and sides comprisingside chines protruding downward from the flattened bottom, wherein saidflattened bottom is not inclined upwards.

Briefly, the hull according to the present invention has a bottomhorizontal or inclined downward from the midship cross section up to thestern according to three variants of the embodiment of the invention,and the choice of either variant depends on the field of use of the boataccording to the invention.

The architecture of the hull, with its deep V-shaped keel and sidesprovided with side chines or skegs protruding downward, gives the boatstability and maneuverability that reduce the negative impact of a lowdead-rise angle substantially on most of the boat sections.

BRIEF DESCRIPTION OF DRAWINGS

Further features and advantages of the present invention will becomemore apparent from the indicative, and therefore non-limitingdescription of a preferred but not exclusive embodiment of a hull forlow drag boats and of variants thereof, as illustrated in theaccompanying drawings in which:

FIG. 1 shows the behavior of a known hull, in a drawing made from aphotograph of a test performed in a Froude's tank;

FIGS. 2 and 3 are drawings from photographs that depict a complete viewand a partial view limited to the stern respectively, of a model testedin late 2011 in the Froude's tank I. N. S. E. A. N. of the Consiglionazionale delle ricerche (CNR, National Research Council) in Rome, themodel corresponding to a 70 t and 24 m vessel according to the inventionat 24 knots;

FIG. 4 is a schematic construction plan of the embodiment of the hull inFIGS. 2 and 3;

FIG. 5 is a schematic side view of the embodiment of the hull in FIG. 4;

FIG. 6 is a cross section of a first variant of the hull in FIGS. 2 and3;

FIG. 7 is a schematic side view of a second variant of the hull in FIGS.2 and 3; and

FIG. 8 is a schematic side view of a third variant of the hull in FIGS.2 and 3.

DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

A schematic representation of the hull according to the presentinvention is shown in FIGS. 4 and 5, which are a schematic plane of theconstruction of a first embodiment of the hull, and a schematic sideview respectively of the same. In these figures, the different crosssections between the stern indicated as PpAD and the bow indicated asPpAV are marked with reference numerals 1 to 9 on the base. Sides 20have side chines 200 projecting downwards that begin in the midshipcross section marked with 5 and continue towards the stern. The midshipcross section at the waterline separates a front hull from a rear hullpart. The front hull part has a deep V-shaped keel 10 whose dead-riseangle decreases up from the bow to a flattened bottom 30 of the hull inthe aft-most portion of the stern, the flattened bottom extending acrossa majority of a width of said hull at said aft-most stern portion. Therear hull parts has sides 20 including side chines protruding downwardsfrom the flattened bottom 30 that is delimited by the side chines 200.The thickness of the side chines 200 in the sides 20 can be chosenaccording to different design requirements, with the consideration thatthe side chines 200 do not have a floating function either in a staticor in a dynamic condition of the boat. The side chines 200 have only afunction of conveying the flow coming from the bow and containing it inthe stern portion of the boat. Therefore, the side chines 200 can bevery thin like skegs.

The keel 10 extends from the bow towards the stern transforming its ownprofile from a deep V-shape to a flattened bottom 30 that, according tothe invention, extends up to the stern and which is disposedhorizontally, as schematically shown in FIG. 5. Moreover, the flattenedbottom 30 is preferably inclined downward in the last tract between thecross sections 1 and PpAD. It should be understood that the constructionplan according to FIGS. 4 and 5, is not exactly an ideal hull profile,due to problems arising from the scale model. This construction howeversimply indicates a pattern of the bottom in the rear hull part, which isthe main feature of the present invention.

The keel 10 ahead the sides 20 protruding downwards by their side chines200 that limit the flattened bottom 30 has the function of capturing thetransverse waves and the foam associated therewith. Proceeding aft, theside chines 200 increase their immersion in the water and form togetherwith the flattened bottom 30 an inverted channel for containing water,such that the kinetic energy of the water is transformed into pressureenergy.

Reference is made now to FIG. 2, which shows a drawing from a photographof a model with a conformation very similar to that shown in FIGS. 4 and5. The photograph was taken during the tests carried out at the end of2011, in the Froude's tank I. N. S. E. A. N. of CNR in Rome. The modelrepresents a mono-catamaran hull provided with a bottom that remainsgenerally horizontal in its rear part, between the midship cross sectionand the stern. This model corresponds to a 70 t and 24 m vessel at 24knots.

As seen in FIG. 2, in which the conditions of displacement and speedsare the same as the conditions of the boat shown in FIG. 1, thehydrodynamic rise of the bow is balanced by the rise of the sternsection, due to the wave formation and the foam conveyed between theside chines. The trim of the hull remains within about 1 degree withrespect to the horizon. This allows the bow to generate wave formationand foam that are well highlighted in FIG. 2. Since the hull maintainssubstantially the same trim that it would have when stationary, aneffective water flow is allowed from the stern section, as shown in FIG.2, corresponding to the cited photograph, by the lack of wave formationaft of the tested model. This lack of wave formation is evidenced inFIG. 3 corresponding to the cited photograph that shows the flow ofwater coming out aft from the hull bottom as completely flattened. Anintersection line of the side chines 200 with the flattened hull bottom30 is inclined downward aft of amidships, as shown in FIG. 4. Shown inFIG. 6 is a cross section of a first variant of the hull according tothe present invention. According to this variant, sides 21 have sidechines 210 in the form of a boat skeg protruding downwards but recessedwith respect to the sides 21 so that a portion of the flattened bottom30 is limited between the skegs.

This modification is useful to avoid, in an anchored boat, that the sidechines 200, which limit the bottom 30 in the first embodiment, collidewith a pier or a jetty in case of rolling in rough sea. The damage orpartial removal of the side chines 200 protruding downward wouldrestrict their function, with serious consequences to thecontrollability and maintaining the boat's route.

Furthermore, the fact that the side chines 210 are recessed with respectto the sides 21 partially limits the amount of foam that is incorporatedunder the hull, and allows the creation of a housing for the thrusters.

Depicted in FIG. 7, showing a second variant of the present invention,are sides 23 with side chines 230 projecting downward but raising abaftin a portion 24 thereof. According to this variant, the outflow crosssection of water and foam conveyed between the side chines protrudingdownward increases. The advantage is achieved that the outflow speed ofthe water decreases abaft, and accordingly the pressure near the aftcross section of the hull increases. This condition is useful because itimproves the efficiency of the thrusters chosen for a specificarchitecture of the hull according to the present invention. Thethrusters take accelerated water and push it against a high pressurezone.

With reference to FIG. 8, which is a schematic side view of a thirdvariant of the boat hull in FIGS. 2 and 3, one can appreciate that abottom 31 is inclined downward and limited by downward inclined sidechines 230 of sides 23. The side chines 230 have rectilinear low edges.

It should be understood that this third variant of the hull according tothe invention is useful for a boat with improved performances in speedand stability.

The above described features are to be understood as simply referred toan embodiment of the invention and to variants thereof and modificationcan be made without departing from the scope defined in the appendedclaims.

The invention claimed is:
 1. A hull for reducing drag, the hullcomprising: a V-shaped keel (10) defined at a bow of the hull, saidV-shaped keel having a dead-rise angle which decreases up to a flattenedbottom (30; 31) of said hull defined in an aft-most stern portion of thehull, said flattened bottom extending across a majority of a width ofsaid hull at said aft-most stern portion, and sides (20; 21; 22; 23)comprising side chines (200; 210; 220; 230) protruding downward from theflattened bottom (30; 31), forming together with said flattened bottom achannel for capturing transverse waves and foam associated therewith,wherein said flattened bottom (30; 31) is inclined downwards; andwherein said side chines are vertically elongated (200; 210; 220; 230)beginning in a midship cross section of said hull and continuing towardssaid aft-most stern portion.
 2. The hull according to claim 1, whereinsaid side chines (200) prolong said sides (20).
 3. The hull according toclaim 1, wherein said side chines (210) are internally offset from saidsides (21) in a form of skegs.
 4. The hull according to claim 1, whereinsaid side chines (220) of said sides (22) rise upwards at a stern. 5.The hull according to claim 1, wherein said side chines (230) of saidsides (23) have rectilinear edges.